1. Technical Field
The present disclosure relates generally to the logging of subsurface formations surrounding a wellbore using a downhole logging tool, and particularly to making directional measurements for well placement and formation evaluation.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used.
Early logging tools were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions. MWD and LWD tools often have components common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
In one manner of making deep LWD electromagnetic measurements, triaxial antennas carried on a transmitter sub are used in conjunction with a directional receiver (carried on a receiver sub). The relative azimuthal angle between the sub containing the transmitters and the sub containing the receiver usually cannot be controlled when the two subs are made up. Thus, the azimuthal offset between the transmitter and receiver antennas is not normally known. Also, due to technical limitations, the triaxial antennas are not co-located. Such is the current design for tools used in the LWD environment to make deep directional electromagnetic measurements for well placement and electromagnetic look-ahead applications.